Protein phosphorylation is a frequent mechanism of intracellular regulation. A novel class of protein kinases with tyrosine specificity was detected in studies of viral transforming proteins and EGF dependent EGF receptor phosphorylation. This led to the hypothesis that tyrosine kinases were linked to control of growth and development, prompting numerous studies in normal and transformed tissue culture cells. We have investigated alterations of tyrosine phosphorylation in a well characterized model of in vivo growth, rat liver regenartion (LR) following partial hepatectomy. We have demonstrated down regulation of EGF- dependent EGF receptor phosphorylation during LR and shown increased EGF-independent tyrosine kinase activity when hepatocytes are proliferating. We have also studied the mechamism of activation and structure of the rat liver EFG receptor, and generated antiserum against purified rat receptor useful for immunopreipitation. The antisera also has agonist properties stimulating receptor internalization and phosphorylation. To understand the role of tyrosine phosphorylation during growth in vivo we will: 1) Study EGF receptor phosphorylation in intact cells stimulated with EGF or an anti-EGF receptor antibody. We will correlate kinase activation with specific consequences of EGF action that are immediate-altered Ca2+ flux, intermediate- enhanced EGF receptor synthesis, and long term DNA synthesis. 2) Study EGF receptor phosphorylation and membrane localization in normal and transformed hepatic cells transfected with an inducible expression vector carrying the metallothionein promoter and the TGF alpha cDNA. 3)Identify increased tyrosine kinase activity in liver regeneration and compare the p-tyr substrates in regenration with those in EGF-stimulated liver. 4) To determine whether receptor dimers or multimers are intermediates in antibody and EFG-dependent hepatic receptor kinase activitation. 5) Initiate studies of protein kinase C activity and substrates in regenerating liver.